1. Field of the Invention
The present invention relates to an apparatus and a method for crystallization, and, in particular, to an apparatus and a method which can produce large crystals by removing fine crystals by classification.
2. Description of the Background
In the field of crystallization, an apparatus for crystallization is known which can produce crystals having relatively large particle diameters by removing fine crystals using a classification. For example, a draft-tube baffled (DTB) crystallizer, and a Krystal-Oslo crystallizer are known.
A conventional DTB crystallizer is described below with reference to FIG. 1. Reference numeral 1 designates a DTB vessel, and draft tube 2 is disposed across an entire center area within DTB vessel 1. Agitating fan 4 is disposed under draft tube 2, and fan 4 is rotatably driven through shaft 6 by motor 8 provided on DTB vessel 1. The lower portion of the center area of the DTB is open so as to enable it to work as baffle. Outer cylindrical portion 12 is provided at the outer side of the baffle portion 10 of DTB vessel 1 and doughnut-type classification portion 14 is defined between the baffle portion 10 and outer cylindrical portion 12. Classification leg 16 is connected with the lower portion of DTB vessel 1 so as to make the particle diameters of the product crystals uniform.
Steam outlet 18 is provided at the upper portion of DTB vessel 1 and connected with a vacuum pump (not shown) so that the inside of DTB vessel 1 can be adjusted to be less than a predetermined pressure. Classification portion outlet 20 is provided on outer cylindrical portion 12 at the upper portion of classification portion 14, and slurry inlets 22 and 24 are respectively provided on the lower portion of DTB vessel 1 and the lower end of classification leg 16. A slurry recovered from classification portion outlet 20 is heated by a heat-exchanger (not shown), dissolved and fed into the inside of the DTB vessel 1 from the inlets 22 and 24. Slurry outlet 26 is provided at the lower portion of the classification leg 16, and the slurry containing crystals with predetermined particle diameters is discharged from slurry outlet 26.
In the conventional DTB crystallizer of FIG. 1, the slurry fed into the inside of DTB vessel 1 from slurry inlets 22 and 24 is forced to go upward in draft tube 2 by agitating fan 4, is then condensed on an evaporation surface 28 which is a slurry surface, and is forced to go downward along the outer side of draft tube 2 and then circulated inside and outside of draft tube 2. Thus, the crystallization is carried out inside and outside of draft tube 2 so that crystallization portion 30 is formed. The classification is carried out by using the settling in the classification portion 14, and the fine crystals are recovered at classification portion outlet 20. The crystals whose particle diameters are greater than a predetermined value still remain and continue to grow in DTB vessel 1.
Further, since conventional DTB crystallizers can not agitate and circulate the slurry when the slurry contains crystals with large particle diameters, another type of DTB crystallizer has been more recently developed which is provided with another agitating fan disposed outside of the draft tube in addition to the agitating fan disposed inside of the draft tube.
However, since conventional DTB crystallizers agitate the slurry so as to allow no influence from the agitated flow to the classification portion, the crystallization area located in the bottom portion of the DTB vessel can not be sufficiently agitated when the slurry contains crystals with large particle diameters. This is a significant problem which occurs when conventional DTB crystallizers crystallize slurries containing crystals with large diameters.
Further, when the slurry needs to be strongly agitated, such as when the slurry is highly concentrated or the slurry needs to be perfectly mixed, it is more difficult to sufficiently agitate the crystallization area in the vessel. In such a case, since the concentration of the slurry in the crystallization area located in the bottom portion of the vessel becomes high, crystals are deposited which readily causes undesired scalings thereof.
On the contrary, if the slurry is strongly agitated so as to avoid the scaling of the crystals, the classification portion is influenced by the agitation effects and, therefore, it becomes impossible to obtain the classification effects. Thus, it has been difficult to avoid both problems.
Accordingly, it is an object of the present invention to provide an apparatus and a method for crystallization which produce crystals with relatively large particle diameters from a slurry by taking out fine crystals from the slurry using a classification, even when the slurry needs to be strongly agitated, such as when the slurry is highly concentrated or the slurry needs to be perfectly mixed.
The above and other objects are achieved according to the present invention by providing an apparatus for crystallization containing a crystallization vessel, device configured to supply seed crystals of material to be crystallized into the crystallization vessel, device configured to supply feed solution of the material being dissolved into the crystallization vessel, a tube baffle disposed within the crystallization vessel and provided with an opening at a lower side thereof, the tube baffle being provided with a crystallization portion in an inner area and a lower area thereof, the tube baffle and the crystallization vessel defining a classification portion there between, device configured to agitate the seed crystals and the feed solution in a predetermined direction in the crystallization portion, a plurality of buffers disposed within the classification portion so that the buffers vertically extend from the lower end of the classification portion to a predetermined height and are respectively spaced at a predetermined distance from each other, the buffers defining a buffer portion therebetween and a gravitational settling portion thereabove, a classification portion outlet disposed so as to communicate with the upper portion of the gravitational settling portion in the classification portion, device disposed to generate a rising velocity of a slurry containing fine crystals in the gravitational settling portion by discharging the slurry from the classification portion outlet so that the rising velocity of the slurry is equal to or greater than a settling velocity of the fine crystals to be discharged, and device disposed at a lower portion of the crystallization vessel for discharging the crystallized material.